Over the last three months, the U.S. installed more solar panels than in almost any other three-month period in history — thanks mostly to utility companies. From Businessweek:

U.S. solar-panel installations more than doubled in the second quarter from a year earlier led by demand in California, according to the Solar Energy Industries Association.

Installations totaled 742 megawatts in the quarter, up 45 percent since the first quarter, and may reach 3.2 gigawatts by year end, the Washington-based trade group said today in its quarterly market report. California led installations with 217 megawatts, followed by Arizona with 173 megawatts.

The U.S. now has 5.7 gigawatts of installed solar capacity, enough to power 1 million homes, according to GTM Research, a Boston-based consulting company that prepared the report with SEIA.

Graph via the Solar Energy Industries Association. (Click to embiggen.)

The record for installations in one quarter was in the last quarter of 2011, with 791 megawatts installed. The number of panels installed topped the number installed in all of 2009.

The boom was driven by large projects that sell power to utilities, with little growth in residential installations and declines in non-residential projects, Shayle Kann, vice president at GTM, said in an interview Sept. 7. “It’s an indicator that the utility market will be the main story this year and probably for the next few years.”

Graph via the Solar Energy Industries Association. (Click to embiggen.)